Processes for manufacturing flexible wiring boards and the resulting flexible wiring board

ABSTRACT

Against a first resin film formed on a first metal film are pressed bumps on a second metal film so that the bumps are embedded into the first resin film. Either one of the first metal film or the second metal film or both is (are) patterned while the bumps are in contact with the first metal film, and the first resin film is heat-treated while the top of the first resin film is partially exposed to discharge the solvent or moisture from the exposed zone, and cure the first resin film. After curing, the bumps and the first metal film may be ultrasonically bonded to each other. A second resin film and a third metal film may be further layered to form a multilayer structure.

FIELD OF THE INVENTION

The present invention relates to the field of flexible wiring boards,particularly the field of multilayer flexible wiring boards.

PRIOR ART

Double-sided flexible wiring boards having patterned metal films on bothsides of a resin film are widely used because of the high degree offreedom of interconnection.

The metal films on both sides of the resin film are electricallyconnected to each other. Conventional methods for connecting these metalfilms are explained below.

First, the through hole method is explained. Referring to FIG. 7(a), thereference number 110 represents a base material for flexible wiringboards having metal films 112, 113 consisting of a copper foil adheredto the top surface and the bottom surface of a polyimide film 111.

This base material 110 is punched with a drill or the like to form athrough hole 118 as shown in FIG. 7(b). Then, the assembly is carbonizedand then electroplated, so that a copper plating layer 115 grows withinthe through hole 118 and on the surfaces of the metal films 112, 113 toconnect the two metal films 112, 113 via the copper plating layer 115within the through hole 118, as shown in FIG. 7(c).

Secondly, the via hole method is explained. Referring to FIG. 8(a), abase material 120 having a polyimide film 121 adhered on a metal film122 consisting of a copper foil is prepared and an opening 128 is formedin the polyimide film 121 by photolithography (FIG. 8(b)).

Thus, the metal film 122 is exposed at the bottom of the opening 128,and a copper thin film is formed by sputtering on the surface of themetal film 122 exposed at the bottom of the opening 128 and on thesurface of the polyimide film 121 in this state followed byelectroplating to form a copper plating layer 123 on the top surface ofthe polyimide film 121 and the inner face of the opening 128 and the topof the metal film 122 exposed at the bottom of the opening 128. Thiscopper plating layer 123 is connected to the metal film 122 at thebottom of the opening 128.

However, the through hole connection method requires a long time to formmany through holes 118 because through holes 118 are drilled one by one.Moreover, it cannot satisfy the demand for fine patterns because thesize of through holes 118 is limited to about 0.2 mmφ.

The metal film 123 formed by the via hole method is easily separatedbecause of the weak adhesive power between the copper plating layer 123and the polyimide film 121. Furthermore, defects such as pinholes aremore liable to occur in the copper plating layer 123 to make inferiorthe reliability.

An object of the present invention is to overcome the disadvantages ofthe prior art described above and to provide a technique that canconnect metal films without forming any opening.

SUMMARY OF THE INVENTION

In order to attain the above object, the present invention provides aprocess for manufacturing a flexible wiring board, comprising the stepsof forming an uncured first resin film including a solvent on a firstmetal film, pressing bumps on a second metal film against said firstresin film to force said bumps into said first resin film until the topsof said bumps come into contact with said first metal film, thenpatterning at least one of said first or second metal films, andheat-treating said first resin film while the top surface of said firstresin film is at least partially exposed to cure said first resin film.

In one embodiment of the process for manufacturing a flexible wiringboard according to the present invention, said uncured first resin filmis semicured by heating it before said bumps are pressed against saidfirst resin film.

In another embodiment of the process for manufacturing a flexible wiringboard according to the present invention, said semicuring step takesplace at a temperature lower than the boiling point of said solventincluded in said uncured first resin film.

In another embodiment of the process for manufacturing a flexible wiringboard according to the present invention, said semicuring step takesplace at a temperature ranged from 80° C. to 300° C.

In another embodiment of the process for manufacturing a flexible wiringboard according to the present invention, said first resin film issoftened by heating it when said bumps are forced into said first resinfilm.

In another embodiment of the process for manufacturing a flexible wiringboard according to the present invention said curing step is followed byapplying ultrasonic wave to either one or both of said bumps and saidfirst metal film to connect said bumps to said first metal film.

In another embodiment of the process for manufacturing a flexible wiringboard according to the present invention, said step of curing said firstresin film is preceded by patterning either one of said first or secondmetal film and ultrasonic treating the unpatterned metal film and thenpatterning it.

Another embodiment of the process for manufacturing a flexible wiringboard according to the present invention further comprises the steps offorming a second resin film on the top surface of said patterned firstor second metal film, then pressing bumps on a third metal film againstsaid second resin film to force said bumps into said second resin filmuntil they come into contact with said first or second metal film, thenpatterning said third metal film and then curing said second resin film.

In this embodiment, said uncured first resin film may be semicured byheating it before said bumps are pressed against said first resin film.

Another embodiment of the process for manufacturing a flexible wiringboard according to the present invention further comprises the steps offorming a second resin film on the top surface of said patterned firstor second metal film, then pressing bumps on a third metal film againstsaid second resin film to force said bumps into said second resin filmuntil they come into contact with said first or second metal film, thenpatterning said third metal film, then curing said second resin film andthen applying ultrasonic wave to said bumps on said third metal film toconnect said bumps to said first or second metal film.

In this embodiment, said curing step may be followed by applyingultrasonic wave to said bumps on said third metal film to connect saidbumps to said first or second metal film. It is possible to applyultrasonic wave indirectly to said bumps by applying ultrasonic wave tosaid first or second metal film to connect said bumps to said first orsecond metal film. It is also possible to apply ultrasonic wave to bothsaid bumps and said first or second metal film.

In this embodiment, said uncured first resin film may also be semicuredby heating it before said bumps are pressed against said first resinfilm.

The present invention also provides a flexible wiring board comprising aplurality of patterned metal films with a resin film being interposedtherebetween among which adjacent two metal films are electricallyconnected to each other via bumps, wherein said resin film is curedafter said bumps are pressed against the top surface of said resin filmand forced into said resin film to electrically connect said two metalfilms via said bumps.

In one embodiment of the flexible wiring board according to the presentinvention, said resin film is cured by heat-treating it while thesurface of said resin film is at least partially exposed between saidpatterned metal films.

In another embodiment of the flexible wiring board according to thepresent invention, one of said two adjacent metal films connected viasaid bumps is ultrasonically bonded to said, bumps.

According to the present invention as defined above, bumps are pressedagainst a first resin film and forced into the first resin film. Thus,the bumps can be contacted with the metal film underlying the firstresin film without forming any opening in the first resin film. Thefirst resin film is preferably softened by heating it when the bumps areforced into the first resin film.

The bumps may be forced into the resin film by applying ultrasonic waveto the bumps digging or softening the semicured resin film in contactwith the bumps.

When the bumps are embedded into the first resin heated, the first andsecond metal films are adhered to the first resin film. When at leastone of the first and second metal films is patterned in this state toform an opening, the top of the first resin film is exposed at thebottom of the opening.

In this case, the top surface of the first resin film is partiallycovered with the first or second metal film and partially exposed. Whenthe first resin film is heated in this state, the solvent and moistureincluded in the first resin film or the moisture generated during theprogress of the chemical reaction caused by heating is discharged fromthe exposed first resin film so that the first resin film is cured. Thiscuring step gives a double-sided flexible wiring board.

The curing step allows the first resin film to thermally shrink and thebumps to be strongly pressed against the first metal film, whereby thefirst and second metal films are electrically connected via the bumps.

In this case, ultrasonic wave may be applied to cause ultrasonicvibration interface between the bumps and the first metal film after thefirst resin film has been cured, so that the first metal film and thebumps are bonded by ultrasonic vibration energy. Ultrasonic wave may beapplied on either side of the first metal film or the second metal film.

The height of the bumps used for connecting metal films is preferablygreater than the thickness of the first resin film in which the bumpsare to be embedded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(e) is a processing diagram showing a process formanufacturing a flexible wiring board according to the present invention(early steps).

FIGS. 2(a)-(d) is a processing diagram showing a process formanufacturing a flexible wiring board according to the present invention(middle steps).

FIGS. 3(a)-(d) is a flow sheet showing a process for manufacturing aflexible wiring board according to the present invention (late steps).

FIGS. 4(a)-(e) is a flow sheet showing a process for manufacturing aflexible wiring board having a multilayer structure according to thepresent invention (the first half).

FIGS. 5(a)-(c) is a flow sheet showing a process for manufacturing aflexible wiring board having a multilayer structure according to thepresent invention (the second half).

FIG. 6 shows an example of ultrasonic welding apparatus used in theprocess according to the present invention.

FIGS. 7(a)-(c) is a flow sheet showing the conventional through holemethod.

FIGS. 8(a)-(c) is a flow sheet showing the conventional via hole method.

DETAILED DESCRIPTION OF THE INVENTION

An example of flexible wiring board of the present invention and aprocess for manufacturing it will now be described.

First metal film will be described after-mentioned description.

Referring to FIG. 1(a), the reference number 11 represents a secondmetal film consisting of a rolled copper foil having a thickness ofabout 10 μm-20 μm. A carrier film 32 and a photosensitive film 33 areadhered to the top surface and the bottom surface of the second metalfilm 11, respectively (FIG. 1(b)).

Then, the photosensitive film 33 is photographically patterned to forman opening 34 (FIG. 1(c)). The second metal film 11 is exposed at thebottom of this opening 34.

When the assembly is immersed into a plating solution forelectroplating, copper is deposited on the second metal film 11 exposedat the bottom of the opening 34. Copper deposits fill the opening 34.The reference number 21 in FIG. 1(d) represents a bump formed fromcopper deposited in the opening 34.

Then, the carrier film 32 and the photosensitive film 33 are removed toexpose the top surface and the bottom surface of the second metal film11. Bumps 21 stand on the top surface of the second metal film 11 (FIG.1(e)).

Separately from the second metal film 11, a first metal film 12consisting of a rolled copper foil having a thickness of 9 μm-35 μm isprepared (FIG. 2(a)), and a polyimide precursor solution is applied onits surface and heat-treated to form a first resin film 16 consisting ofa polyimide film (FIG. 2(b)).

The heat treatment temperature is lower than the boiling point of thesolvent for the polyimide precursor solution. The heat treatment wascarried out at a temperature (150° C.-200° C.) lower than the boilingpoint 202° C. of the solvent N-methyl pyrrolidone for the polyimideprecursor solution used here. The first resin film 16 in this state hasbeen only slightly imitated and semicured.

Although the first resin film 16 here was formed by applying a polyimideprecursor solution and heat-treating it, a preliminarily semicured resinfilm such as polyimide may be adhered onto the first metal film 12 toform the first resin film 16.

Then, the bumps 21 on the second metal film 11 treated as above arefaced to the first resin film 16 on the first metal film 12 as shown inFIG. 2(c) and hot-pressed to force the bumps 21 into the first resinfilm 16.

FIG. 2(d) shows that the bumps 21 have been forced into the resin film16 so that they are in contact with the first metal film 12 underlyingthe first resin film 16.

Hot-pressing softens the first resin film 16 to help the bumps 21 to beforced into it and induces adhesion on the surface of the first resinfilm 16 to adhere the second metal film 11 to the first resin film 16.

The hot-pressing conditions here are 50 kg/cm² at 150°0 C. and thehot-pressing period is about 10 minutes.

Then, a patterned resist layer is formed on the top surface of the firstmetal film 12 and etched to pattern the first metal film 12. Afteretching, the resist layer is removed to give a flexible wiring board 3having the patterned first metal film 12 (FIG. 3(a)). The referencenumber 35 in FIG. 3(a) represents an opening formed removal zone of thepatterned first metal film 12. The opening 35 is a zone dividing wiringfrom each other. The first resin film 16 is exposed at the bottom of theopening 35. However, the first resin film 16 is not exposed at thebottom surface of the first resin film 16 on the side of the secondmetal film 11.

When this flexible wiring board 3 is heat-treated at a temperature of160° C.-350° C. in a baking apparatus for several hours, the residualsolvent included in the first resin film 16 is discharged into theatmosphere from the exposed first resin film 16 at the bottom of theopening 35 in the patterned first metal film 12. This heat treatmentdegases the first resin film 16 and promotes imidation reaction withinthe first resin film 16 to cure the first resin film 16. The moisturegenerated during imidation reaction is discharged from the exposed firstresin film 16 by heat treatment.

Once the first resin film 16 is cured by this imidation reaction, thefirst and second metal films 12, 11 are fixed to the first resin film16. During then, the first resin film 16 thermally shrinks and the bumps21 are pressed against the first metal film 12, whereby the first andsecond metal films 12, 11 are electrically connected via the bumps 21.

Then, the bumps 21 and the first metal film 12 are ultrasonically bondedto enhance the reliability of their electric connection.

The reference number 50 in FIG. 6 represents an ultrasonic bondingapparatus used for this ultrasonic bonding.

This ultrasonic bonding apparatus 50 comprises a platform 56, two guideposts 57 ₁, 57 ₂ upright on the platform 56, an ultrasonic wavegenerator 51 supported to be vertically movable by the guide posts 57 ₁,57 ₂, and a resonator 52 attached to an end of the ultrasonic wavegenerator 51.

A working table 58 is placed on the platform 56 and a flexible wiringboard 3 imitated as described above is mounted on the top of the workingtable 58.

When a planer tip 54 of the resonator 52 is positioned in parallel tothe surface of the working table 58 and an air cylinder 53 of theultrasonic bonding apparatus 50 is activated so that the ultrasonic wavegenerator 51 and the resonator 52 vertically descend along the guideposts 57 ₁, 57 ₂. The tip 54 of the resonator 52 comes into closecontact with the flexible wiring board 3.

This state is shown in FIG. 3(b), in which the tip 54 of the resonator52 is pressed against the flexible wiring board 3 by the air cylinder 53so that the tops of the bumps 21 are strongly pressed against the firstmetal film 12 because the first resin film 16 is softer than the firstand second metal films 12, 11 and the bumps 21.

When the ultrasonic wave generator 51 is activated in this state toapply ultrasonic wave to the resonator 52, the ultrasonic wave resonateswithin the resonator 52 so that the tip 54 of the resonator 52ultrasonically vibrates. This ultrasonic vibration causes rubbinginterface between the first metal film 12 and the bumps 21, whereby thetops of the bumps 21 are metallically bonded to the first metal film 12.In this case, preliminary solder plating on the bumps 21 furtherfacilitates bonding.

The flexible wiring board 3 is removed from the ultrasonic bondingapparatus 50 and a patterned resist layer is formed on the top surfaceof the second metal film 11, which is then etched. After etching, theresist layer is removed. The reference number 36 in FIG. 3(c) representsan opening formed in the patterned second metal film 11.

An overcoat solution is applied on the surfaces of the first and secondmetal films 12, 11 of this flexible wiring board 4 and polymerized intofilm to form overcoat layers 25, 26, whereby a double-sided flexiblewiring board 5 is obtained. Other electronic components can be connectedto the first and second metal films 12, 11 exposed from openings notshown formed in predetermined zones of the overcoat layers 25, 26.

Although overcoat layers 25, 26 may be formed to prepare a double-sidedflexible wiring board 5, the flexible wiring board 4 having the firstand second metal films 12, 11 exposed can also be used to prepare aflexible wiring board having a multilayer structure.

FIG. 4(a) shows a flexible wiring board 4 having the first and secondmetal films 12, 11 exposed on the top surface and the bottom surface ofthe first resin film 16. (This flexible wiring board 4 is the flexiblewiring board 4 shown in FIG. 3(c).) A polyimide precursor solution isapplied on the top surface of the flexible wiring board 4 andheat-treated to form a second resin film 18 consisting of a polyimidefilm shown by the reference number 18 in FIG. 4(b). This second resinfilm 18 has not been imitated.

A third metal film 13 having bumps 22 is prepared, and the bumps 22 arefaced to the second resin film 18 (FIG. 4(c)) and brought into contactwith the second resin film 18 and hot-pressed, whereby the bumps 22 areforced into the second resin film 18 until the tops of the bumps 22 comeinto contact with the patterned first metal film 12. During then, thethird metal film 13 is bonded to the second resin film 18.

Then, a patterned resist layer is formed on the top surface of the thirdmetal film 13, which is then patterned by etching.

The reference number 37 in FIG. 4(e) represents an opening in thepatterned third metal film 13. The assembly is heat-treated while thetop surface of the second resin film 18 is exposed at the bottom of thisopening 37 under the same conditions as above to discharge the solventand moisture from the opening 37 and thus imidate the second resin film18.

This imidation allows the third metal film 13 to be fixed to the secondresin film 18, which thermally shrink to press the bumps 22 against thefirst metal film 12, whereby the first and third metal films 12, 13 areelectrically connected via the bumps 22. Thus, the first to third metalfilms 12, 11, 13 are electrically connected via the bumps 21, 22.

Then, the flexible wiring board 6 in this state is mounted on theworking table 58 in the ultrasonic bonding apparatus 50 shown in FIG. 6and brought into contact with the tip 54 of the resonator 52. When theultrasonic wave is applied, the bumps 22 ultrasonically vibrate and areultrasonically bonded to the first metal film 12 in contact with them.After ultrasonic bonding, the flexible wiring board removed from theultrasonic bonding apparatus 50 has a multilayer structure shown by thereference number 7 in FIG. 5(b).

A polyimide precursor may be applied on this flexible wiring board 7 toform a resin film, which may be further layered on a metal film havingbumps and imitated. In this case, the steps shown in FIGS. 4(b)-(e) andFIG. 5(a) are repeated.

An overcoat solution may be applied on the top and the bottom of thisflexible wiring board 7 and cured to form overcoat layers 27, 28 on thesecond and third metal films 11, 13.

As has been described above, the present invention can simplify theprocess because patterned metal wiring films can be connected via bumpswithout preliminarily providing openings in the polyimide film.

The present invention also improves reliability because bumps andpatterned metal wiring films are steadily electrically connected usingan ultrasonic bonding apparatus.

Although the multilayer flexible wiring board 8 was formed by two stepsof ultrasonic bonding according to the foregoing embodiments, the firstto third metal films 11, 12, 13 may also be connected via bumps 21, 22by a single application of ultrasonic wave.

Solder coat or gold coat may be formed on the bump tops to facilitateultrasonic bonding.

Although the polyimide precursor solution used for forming the first orsecond resin film 16, 18 included N-methyl pyrrolidone as a solvent inthe foregoing embodiments, polyimide precursor solutions including othersolvents such as formalin or N-methylamide may also be used.

Instead of polyimide precursor solutions, liquid raw materials of otherresins may also be used, such as liquid raw materials of modified epoxyresins, liquid raw materials of polyester resins or liquid raw materialsof polyethylene resins.

When polyester liquid raw materials or the like include an organicsolvent, a semicuring step can take place by heating to a temperature ator below the boiling point of the organic solvent.

Alternatively, a semicured resin film such as a modified epoxy resin,polyester resin or polyethylene resin may be used.

Even if a non-polyimide resin liquid raw material is used to form firstand second resin films 16, 18 or a semicured resin film is adhered toform first and second resin films 16, 18, it is also preferable todischarge either one of the organic solvent or moisture or both from thepartially exposed resin film surfaces when the resin films are cured byheating after the bumps are forced into the semicured resin films.

In brief, the present invention widely includes processes formanufacturing a flexible wiring board, comprising the steps of pressingbumps into a uncured or semicured resin prior to a curing step toconnect metal films on the top surface and the bottom surface of theuncured or semicured resin film via the bumps, and heat-treating thepartially exposed resin film to cure it.

Although the metal films described above consisted of copper, othermetals may also be used. Gold or other plating coats may be formed onmetal films.

Although the resin film 16 was cured before the second metal film 11 waspatterned in the foregoing embodiments, the first resin film 16 may alsobe cured after the first and second metal films 12, 11 have beenpatterned.

On the contrary, the first metal film 12 may be patterned and then thebumps 21 on the second metal film 11 may be pressed into the first resinfilm 16 on the top of the first metal film 12 to cure the first resinfilm 16 in this state. In brief, metal films in contact with bumps maybe in the form of a metal foil or a patterned wiring film.

The process can be simplified by contacting bumps with a metal filmunderlying a resin film without forming any opening.

Moreover, electric reliability can be improved by applying ultrasonicwave to bumps and metal films in contact with the bumps to connect themby ultrasonic vibration energy.

What is claimed is:
 1. A flexible wiring board comprising a plurality ofpatterned metal films with a resin film being interposed therebetween,among which at least adjacent two patterned metal films are electricallyconnected to each other via bumps, one of said patterned metal filmhaving at least one opening, wherein said resin film includes a solventand the resin film is thermally shrinkable to press said bumps againstsaid at least adjacent two patterned metal films by discharging thesolvent through at least one opening of said patterned metal film whenheat is applied to the flexible wiring board.
 2. The flexible wiringboard according to claim 1 wherein said resin film is cured byheat-treating it while the top surface of said resin film is at leastpartially exposed between said patterned metal films.
 3. The flexiblewiring board according to claim 1 wherein one of said two adjacent metalfilms connected via said bumps is ultrasonically bonded to said bumps.4. The flexible wiring board according to claim 2 wherein one of saidtwo adjacent metal films connected via said bumps is ultrasonicallybonded to said bumps.
 5. The flexible wiring board according to claim 1wherein said thermally shrinkable resin film shrinks to press said bumpsbetween said two patterned metal films to electrically connect said twopatterned metal films via said bumps.
 6. A flexible wiring boardcomprising a first metal film, a second metal film, at least one bumpbetween the first and second metal films to electrically connect thefirst and second metal films, and means for pressing the at least onebump against at least one of the first and second metal films byshrinking when heat is applied to the flexible wiring board.